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calculation of PEP ?

Mar 20th, 10:04

AB3FN

Joined: Jun 5th 2007, 14:11
Total Topics: 0
Total Posts: 0
I'm missing something basic here, and hope somebody can educate me.

Consider a typical SSB speech waveform. If we have PEV (peak envelope voltage) and want to calculate PEP (peak envelope power), we first convert the PEV into an RMS value. Then the RMS value is squared and divided by R to get PEP. So PEP really refers to the average power, and not to the true peak power.

Is this simply a peculiarity with terminology, or am I missing something fundamental about the calculation?

A quick search finds references to PEP calculations based on both the PEV and on the RMS value. Both cannot be correct.

Thanks in advance for your response.

Mar 21st, 10:00

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
SSB is modulated onto a carrier. Thus, it makes sense to average the power over one cycle of the carrier.

Zak W1VT
ARRL Senior Lab Engineer
Mar 21st, 10:42

AB3FN

Joined: Jun 5th 2007, 14:11
Total Topics: 0
Total Posts: 0
Thanks, Zak, but I still don't grasp this.

Let's suppose that the modulating signal was not speech, but a constant amplitude single tone. The SSB waveform would be a clean sine. Would we still use the RMS value to calculate PEP ? I understand that average envelope power may generally be a more useful figure for us, so perhaps calling it peak envelope power is just a misnomer ? And if we're designing a circuit to handle true peak power, regardless of the waveform or duty cycle, then shouldn't we be concerned with the true peak voltage ? For signals other than speech modulated SSB, e.g. FM, is PEP normally calculated from true PEV or RMS ?

My apologies for belaboring this, but thanks in advance for the education.
Mar 21st, 14:07

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
You look at the modulation envelope to determine the peak value. Then you can zoom in (in terms of time) and then calculate the power for one RF cycle.

FM and CW are continuous duty cycle modes. If you apply 1500 watts of either to a dummy load, the thermal rise obviously needs to be the same as 1500 watts of DC applied to the dummy load. Calculate the voltage that needs to be applied to a 50 ohm dummy load in both the DC and RF cases..

Zak W1VT
ARRL Senior Lab Engineer
Mar 27th, 14:20

KE0ZU

Joined: Jul 16th 2017, 08:15
Total Topics: 0
Total Posts: 0
PEP is simply that, the power present at the instant of a particular "peak" of the speech/RF envelope.

Every speakers voice is different and on "average" contains a different power level. So the same will be true for the "average" power of an SSB envelope. Compression increases the "average" speech power of a signal, while clipping "limits" the peak power.

As Zak mentions FM and carrier frequency shift RTTY are average power emissions, with no variation in amplitude, so the RMS value of each will equal their DC equivalents.

CW, although a constant power level during carrier on, does vary somewhat in average power. Also this affects the power dissipation of an amplifier tube somewhat differently than a SSB signal, which is why there may be a differentiation in permissible power levels.

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